I'm getting a vertical slicer on the textures where the tree isn't rendered. The problem is more evident in animations, where the forest trees are turning sideways then back to facing the camera.
This problem is due to a precision error in the BSP algorithm of Mental Ray. It happens only under certain conditions:
This is not specific problem of Forest. The rendering artefacts can be reproduced using a sample mesh, that includes two very closed planes
There are two possible workarounds for this issue: